Glass fibers which are produced on a commercial scale today in the manufacture of continuous glass strands are typically either "E" glasses or "621" glasses. Most of the continuous glass fibers made today are glasses of the "E" glass type with the balance primarily being of the "621" type.
"E" glass is generally described and claimed in U.S. Pat. No. 2,334,961. These glasses are typified by the composition listed below in Table I with the constituents being in weight percent:
Table I ______________________________________ Ingredients Percent ______________________________________ SiO.sub.2 52-56 Al.sub.2 O.sub.3 12-16 CaO 16-19 MgO 3-6 B.sub.2 O.sub.3 9-11 ______________________________________ " 621" glass is a modification of a typical "E" glass formulation such as shown above and is typically devoid of MgO, with a CaO content much higher than an "E" glass. The "621" glasses are described in more detail in U.S. Pat. No. 2,571,074. In general these glasses have a composition as shown in Table II, the percentages shown being by weight:
Table II ______________________________________ Ingredients Percent ______________________________________ SiO.sub.2 52-56 Al.sub.2 O.sub.3 12-16 CaO 19-25 B.sub.2 O.sub.3 8-13 ______________________________________
As will be readily understood by the skilled artisan, minor constituents are present in both "E" and "621" glasses. These constituents are typically F.sub.2, Fe.sub.2 O.sub.3, K.sub.2 O, Na.sub.2 O and MgO and on occasion BaO. In general these minor constituents are present each in amounts of less than 1 percent by weight in the glass.
As will readily be appreciated from an analysis of the above formulations, the elimination of magnesium oxide in the "621" glass permitted the use of lower quantities of B.sub.2 O.sub.3 in an "E" glass, but required the addition of substantially more calcium oxide and the absence of magnesium oxide.
While both of these glasses have achieved considerable success in the continuous glass fiber field, it is still apparent that considerable quantities of B.sub.2 O.sub.3 are still required. In recent years the cost of boric acid and colemanite ore, another source of boric acid, has risen steadily causing overall batch costs in the manufacture of glass fibers to increase substantially. While all batch costs have in general increased, the boric acid constituent has had a particularly significant effect on the increased cost of glasses for the manufacture of continuous glass fibers. It is thus desirable to reduce the quantities of B.sub.2 O.sub.3 in glasses used to make continuous glass fibers while still adhering to the properties of the "E" and "621" glasses.
In providing a lower B.sub.2 O.sub.3 content in the glass used to manufacture glass fibers, substantial reduction in batch costs are achieved. The other batch constituents are adjusted to compensate for the B.sub.2 O.sub.3 reduction and a small quantity of MgO is usually present in the novel glass of this invention.